1. Field of the Invention
The invention relates to mine roof bolt assemblies and more particularly, to washers, bushings, gaskets and the like for sealing the interiors of mine roof bolt holes from ambient air present in mine shafts.
2. Description of the Prior Art
It has been found that certain layers of mine roof strata having a sufficient content of water absorbent clays such as montmorillonites, illites, etc., are very vulnerable to reaction with humid mine air. It has also been found that layers of roof strata containing certain minerals such as pyrite, calcite, etc. react adversely with either oxygen or a combination of moisture and oxygen and become greatly weakened by such reaction. If the roof bolt hole is not reliably sealed from the mine atmosphere, the roof bolt hole exposes the above-mentioned kinds of layers in the roof strata to a continually replenished supply of oxygen and/or water vapor, as subsequently explained. A vapor phase transfer reaction occurs wherein the moisture of the air with the above-mentioned water absorbent clays, or the reaction of the oxygen with the above-mentioned pyrites, calcites, etc., causes the corresponding layers of upper strata to weaken or oxidize. It has been found that as long as the supply of moist air and oxygen is replenished, the deterioration of the above kinds of layers extends laterally, eventually severely degrading the strength of the "laminated" mine roof strata configuration by causing lateral "slippage" between layers on either side of the degraded layer. The load bearing characteristics of such roof strata are believed to be greatly weakened even though the mine roof bolts remain tightly anchored in the upper strata. Diurnal barometric pressure changes, seasonal atmospheric pressure changes, pressure variations caused by pulsations of mine ventilation systems, variation in pressure caused by shooting and blasting, and variations in pressure due to extending and closing of working areas to mine ventilation all cause pressure changes in the ambient mine atmosphere which causes a "breathing" type of air exchange between the interior of roof bolt holes and the outside mine atmosphere if the roof bolt assemblies do not provide airtight sealing between the interior of the roof bolt hole and the mine.
Many of the known roof bolt assemblies do not provide reliable airtight sealing to prevent the above-mentioned "breathing," thus leaving the various above-mentioned strata exposed within the roof bolt holes and vulnerable to the above-described degradation due to the action of moisture and oxygen "inhaled" by the roof bolt holes. Therefore, a more detailed description of the action of the roof shales which are vulnerable to moisture is appropriate.
It has been found that shales are of sedimentary origin and composed of a mixture of fine grained sediments deposited in a layered type formation. The shales consist of hardened clays and silts possessing cleavage parallel to bedding. After deposition of the sediments the change from clay to shale is attended by a greater or less degree of recrystallization of the constituents and usually some enlargement of the particles. Many shales include a percentage of water absorbing clays such as the montmorillonite (smectite) group and to a lesser degree the illites, hallosites, etc., of the clay family of minerals. The clays may be finely dispersed throughout the shale formation or they may be deposited in very thin layers within the formation. The clay may vary in the concentration and location both vertically and laterally within the shale formation. A general observation of the shale roof strata of coal seams in the Midwest is that several inches of roof strata immediately above the coal, usually consisting of a dark siltstone (miner's slate) is more impervious to moisture than the upper portions of the immediate mine roof shale. The color of the roof shale is not a reliable indicator of the presence or absence of water absorbing clays.
The clays are very fine grained sedimentary deposits and generally consist of hydrated silicates of aluminum with various impurities. The water absorbent clays such as the montmorillonites, upon absorbing water, expand in volume and become plastic, which destroys their ability to resist stresses of both compression and tension. These clays have a high cation exchange capacity as compared to the less absorbent clays, they also have a flat pate-like crystalline structure. It is the interlayer water between the unit silicate layers of these minerals that causes their swelling and expanding characteristics. One important characteristic of the montmorillonites is that they absorb water up to a given point where equilibrium is reached, however, if allowed to dry, even a small amount, they will slake and swell immediately if again exposed to water. For this reason, it is important that bolt holes be sealed promptly against the drying of inherent and stabilized moisture of the clay and the reintroduction of moisture from the air. Clays that are water absorbent may expand as much as 1200% of their volume, and when confined, exert pressures in excess of 14,000 p.s.i.
Surface air taken into the mine contains moisture in the form of water vapor (gaseous state of water). The relative humidity of the air is measured as the ratio of the amount of water vapor actually present to the greatest amount possible at a given temperature. The total amount of vapor possible in the air varies with the temperature, the higher the temperature, the greater the capacity for holding vapor in the air. On a very humid day, the vapor content may be as much as 7% or more of the air. Water vapor expands in air space completely, equalizing the humidity of air within a bolt hole with the humidity of the air within the mine workings (Dalton's law of partial pressures and the kinetic molecular theory). Since the transfer of water from air to the clay in the shale takes place in a vapor phase transfer, restrictions to the entrance of mine air into a bolt hole can affect the volume and rate of vapor transfer into the bolt hole, and in turn, the time period involved in the clay-water reaction. The humidity of the air is constantly changing, during the day, in cycles, from day to day, and seasonal changes, the summer months being more humid than the winter months in the Central and Eastern United States.
A variety of mine roof bolt assemblies are known in the art. They usually include a bolt of from three to six feet in length, a roof plate or support plate through which the roof bolt extends, and an expansion shell threaded onto a threaded end of the roof bolt. A mine roof bolt hole is drilled, usually perpendicularly to the surface of the mine roof, with the expansion shell inserted into the roof bolt hole such that the support plate abuts the mine roof. The roof bolt is tightened, causing the expansion shell to expand, thereby anchoring the entire assembly into the mine roof strata and forcing the support plate inwardly against the mine roof. The mine roof strata described above is composed of various layers of different types of rock having varying strength characteristics. A plurality of spaced mine roof bolts installed in the mine roof tends to secure the various layers of mine roof strata together to prevent "slippage" therebetween, increasing the strength of the laminated strata, thereby preventing caving of the mine roof. However, up to now, the known mine roof bolt assemblies have not satisfactorily supported mine roofs wherein mine roof strata exposed by the roof bolt holes to ambient mine air and moisture has caused certain kinds of mine roof strata to weaken. Mine roof bolt assemblies are known to fall out of degraded roof bolt holes, eliminating the strata layer binding needed to prevent slippage between different layers of mine roof strata. It is also known that in certain cases, a mine roof may collapse even though all of the mine roof bolts therein are sufficiently tightly anchored in a hard layer, such as limestone.
This may be due to the fact that when a roof bolt hole is drilled into a shale roof it exposes the interior of the hole to mine air. Mechanical expansion shell-type bolt assemblies are not designed to provide airtight sealing of the bolt hole. When the roof bolt assembly is tightened in the hole, mine air can enter the hole, through the opening in the base plate and around the shank of the bolt and over the top of the base plate, particularly when the bolt is not installed perpendicular to the roof surface, or when the exposed roof surface is uneven. Vapor from humid mine air enters the bolt hole and comes in contact with the exposed shale strata in the interior of the bolt hole. If the exposed strata contains water absorbent type clays, then the vapor will react with the clay causing a deterioration of the strata. It is my belief that the progressive deterioration of the vulnerable strata takes place in cycles, viz the clay reacts with the water vapor until it reaches its equilibrium point, then a decrease in humidity permits a partial drying of the affected clay, then upon the next increase in humidity, the clay rapidly absorbs water until it again reaches its equilibrium point. The expansion and contraction of the clay in cycles affords an opportunity to extend the perimeter of the interface zone between the changed (by water) crystalline structure of the clay and the undisturbed crystalline portion of the clay formation. These water-clay cycles may be frequent or extended depending upon the variation in the humidity of the air, and the distribution of the water absorbent clays exposed in the bolt hole. As the affected (disturbed) area of the clay works its way outward from the hole, the rate of extension initially is greatly retarded due to the exponential increase in the area involved.
In a mine roof where the bolt holes are not sealed airtight, the vulnerable shales exposed within the hole interact with vapor from the air, the clay expands and becomes plastic and the stress resistance of the disturbed formation is destroyed, this causes a shift in the stress load carried by the roof strata which may cause overstressing in other areas of the mine roof. The strata beneath the disturbed clay zone may fail, unless the rock resistance of the lower strata is sufficient to offset the lost support of the disturbed upper zone. If the affected area is in the area of contact between the expansion shell and the wall of the bolt hole, the plasticity of the affected clay will relieve the stress imposed on the bolt hole wall by the expansion shell and the shell will become loosened and ineffective. If the area affected is below the area of contact with the expansion shell, then the strata below the affected zone may fall away from the bolt leaving the bolt dangling in the remaining portion of the roof. Where the upper part of a roof fall coincides with the top or upper part of the bolt holes, it is indicative that the cause of the fall may be due to the interaction of the humid mine air with the clay of the strata intersected by the bolt holes.
Many roof falls in shale roof in bolted areas are unpredictable on account of the many variables involved, including (1) the amount and location of water absorbent clays present in the roof shales, (2) the type of clays present, (3) restrictions to the entrance of mine air into the bolt holes due to the configuration of the surface of the roof and the positioning of the bolt and base plate with respect to the hole opening, and (4) the relative humidity of the mine air, particularly the seasonal variations in humidity.
U.S. Pat. No. 2,829,502 describes a mine roof bolt assembly for excluding mine air from a mine roof bolt hole to prevent spalling or crumbling of the side walls of the interior of the roof bolt hole by use of a large conical stopper-like washer on the shaft of a particular type of roof bolt. U.S. Pat. No. 3,651,651 shows a stabilizing bushing and a flat washer. U.S. Pat. No. 3,521,454 illustrates a flexible annular washer which accommodates variations in the surface surrounding the mount of a rock bolt hole. U.S. Pat. Nos. 4,183,699 and 4,188,158 disclose seals which contact the mine roof at the mouth of a roof bolt hole. In some instances, irregularity of the material around a roof bolt hole prevents these seals from being completely effective. The state of the art in roof bolt assemblies is believed to be further indicated by U.S. Pat. Nos. 4,162,133; 3,528,253; 4,103,498; 4,147,458 and 3,238,731. Thus, none of the known rock bolt or roof bolt assemblies provide reliable sealing of the interior of the mine roof bolt hole from mine air in certain practical instances, including the one set forth below.
Mine roof bolt holes ordinarily are drilled to a closed hole diameter tolerance in order to provide effective anchoring by means of the above-mentioned anchor shell assembly of conventional roof bolt assemblies. In certain instances, the roof strata is softer at the base portion than in the upper part whereat the anchor shell is to engage the strata comprising the walls of the roof bolt hole. Where such soft strata at the base portion of the hole is encountered, the drill bit utilized for drilling a standard diameter roof bolt hole rapidly penetrates the soft strata, making an irregularly shaped hole having a corkscrew-like pattern. Such holes, drilled by standard diameter drill bits in soft strata, usually are somewhat undersized. Consequently, considerable difficulty is frequently experienced in inserting the anchor shell of a roof bolt assembly into the lower portions of such roof bolt holes. In order to overcome this difficulty, it is common practice to first drill the lower part of the roof bolt hole in the soft strata with a drill bit having a diameter that is slightly oversized, and then change to the standard diameter drill bit as firmer strata into which the anchor shell will be anchored is encountered. The upper portion of the roof bolt hole is then drilled utilizing the standard diameter drill bit.
Although the foregoing technique avoids the difficulty of inserting the anchor shell into the roof bolt hole, it complicates sealing of the roof bolt hole to the roof bolt shafts, which, as explained above, is necessary to prevent deterioration in the strata to which the roof bolt is anchored. The complication referred to is the presence of substantial numbers of both standard diameter and oversized diameter roof bolt holes, requiring the use of at least several diameter roof bolt seal assemblies to effect the necessary sealing of the lower portion of the roof bolt hole from ambient mine air.
Therefore, it is an object of the invention to provide a bushing/seal assembly for sealing a roof bolt and roof bolt hole, which bushing/seal assembly effectively seals a roof bolt hole having any diameter within a predetermined range.
Although roof bolts are ordinarily axially aligned with the roof bolt holes when the roof bolt assemblies are inserted therein, anchoring of the anchor shells frequently result in "tilting" of the tightened roof bolt shaft within the roof bolt hole, causing a lower portion thereof to be "offset" or off-center with respect to the axis of the roof bolt hole. This condition frequently causes difficulty in obtaining an airtight seal of the roof bolt hole.
Therefore, another object of the invention is to provide a bushing/seal assembly for a roof bolt which avoids a sufficient amount of tilting of the roof bolt shaft after anchoring of the roof bolt assembly to prevent destruction of a seal of the roof bolt hole by means of a seal assembly through which the roof bolt shaft extends.